This is fascinating. Px is an angular measurement, convertible from radians.
The moon from earth is about 24.3px wide, for example. The Eiffel Tower from a mile away is about 550.5px tall. That means that when you do “{ width: 24.3px }” in CSS 2.1, you're making something as wide as the moon looks to be.
All these years of web slinging and I always thought it meant "pixels".